A typical example of a storage apparatus provided with a magnetic recording medium may be a HDD (Hard Disk Drive) having a magnetic disk. The HDD is not only used in computers, but also in video recording apparatuses, portable music players, and the like. Because the HDD is used in various kinds of apparatuses, there are demands to further increase the storage capacity and to further reduce the size of the HDD.
In order to increase the storage capacity and reduce the size of the HDD, it is necessary to improve the recording density of the magnetic disk. Conventionally, the magnetic disk generally employs the longitudinal magnetic recording system that orients an axis of easy magnetization of a recording layer in a direction parallel to a substrate surface. On the other hand, the recently developed magnetic disk employs the perpendicular magnetic recording system that orients the axis of easy magnetization of the recording layer in a direction perpendicular to the substrate surface. In the perpendicular magnetic disk, the effects of demagnetization is small at recording bit boundaries of the recording layer, and thus, a high-density recording of data is possible because the data may be recorded sharply. In addition, perpendicular magnetic disks have been proposed (Patent Documents 1 and 2) in which a back layer formed by a soft magnetic material is provided under the recording layer in order to further improve the recording density.
The general perpendicular magnetic disk has a structure in which a soft back layer having a thickness of approximately 30 nm to 100 nm, a seed layer, an intermediate layer, a granular layer formed by a Co-based alloy and an oxide, a Co-based alloy layer including no oxide, a carbon protection layer, a lubricant layer, and the like are stacked on a non-magnetic substrate made of glass or the like. In the HDD provided with the perpendicular magnetic disk, a magnetic head is used to record data on the perpendicular magnetic disk. Magnetic flux generated from the magnetic head penetrates the recording layer perpendicularly towards the back layer, and magnetizes the recording layer in the perpendicular direction.
In a granular recording layer, a non-magnetic oxide material is formed at grain boundaries of magnetic crystal grains and magnetically separates or isolates the magnetic crystal grains, to thereby reduce medium noise. The granular recording layer is sometimes also referred to as a recording layer having a granular structure.
In order to further improve the recording density of the perpendicular magnetic disk, it is necessary to improve the signal quality of the data recorded on the perpendicular magnetic disk, such as the error rate and the SNR (Signal-to-Noise Ratio). Improving the performance of the recording layer is important in improving the signal quality of the data recorded on the perpendicular magnetic disk. More particularly, it is necessary to reduce the size of the magnetic crystal grains, make the magnetic crystal grain sizes uniform, and reduce the variation in the crystal orientation of the magnetic crystal grains in the recording layer. For this reason, the size of the magnetic crystal grains is reduced, the magnetic crystal grain sizes are made uniform, and the variation in the crystal orientation of the magnetic crystal grains is reduced, by taking measures such as forming the intermediate layer by a Ru layer having a physical segregation structure, forming the recording layer by a granular recording layer including an oxide, and further making the intermediate layer or the recording layer have a multi-layer structure or providing a non-magnetic granular layer. However, in order to further improve the recording density, it may be desirable to further reduce the size of the magnetic crystal grains, make the magnetic crystal grain sizes more uniform, and further reduce the variation in the crystal orientation of the magnetic crystal grains.